Solar energy is converted into electrical energy using a photovoltaic (PV) cell. Banks of such cells are often deployed together as a PV array. The electrical output of a PV array is typically fed into an AC supply grid. The direct current (DC) voltage of the array is converted into the alternating current (AC) voltage of the supply grid by a bulk inverter or grid-tie inverter. The grid-tie or bulk inverter is used to make the electrical power supplied to the grid of the correct frequency and voltage. A known range of grid-tie inverters is manufactured by Control Techniques of Newtown, Powys, Wales. Electrical energy is also sometimes supplied to a DC storage network instead of an AC grid.
For a given level of irradiation (exposure to the sun) and temperature, each PV cell and PV string, panel or array has an optimal DC operating voltage which can be found and followed using an automated maximum power point (MPP) tracking algorithm running in an associated power processing system.
An MPP (Maximum Power Point) tracker is a standard requirement in all PV systems whether it is resident in a bulk grid-tie power inverter or in discrete DC-DC converters at the end of each string of PV cells within a solar array. Its purpose is to track the optimal power point for the solar array's temperature and the level of irradiation coming from the sun, as a result of which the DC voltage from the solar array is regulated, in order to get the best overall power from the system.
A decrease in temperature will increase the optimal DC voltage (MPP VDC) calculated by an MPP tracker for a solar cell, string, array or panel, as will an increase in irradiation. Conversely an increase in temperature or a decrease in irradiation will reduce the MPP VDC. When the irradiation level changes by a large amount, this will only cause a relatively small change in MPP VDC.
FIG. 1 shows the relationships between the DC voltage provided by a typical solar array (VDC) and its current (IDC) and power (WDC). As can be seen from FIG. 1, the relationships (or so called “power characteristic”) for a typical solar array are not linear, and therefore a simple PID controller cannot be used to control voltage across the array since, depending on the present value of VDC, a positive increase in voltage could lead to either a positive or negative change in power (WDC). To solve this problem a common industrial technique called Perturb and Observe (P&O) is conventionally employed, whereby a following error term is derived by modulating a test pattern on top of the regulated VDC level for a solar array, in order that power can be measured when the modulated VDC is positive and negative with respect to the present MPP VDC.
FIG. 2 shows the effect of a modulated test pattern being applied to the regulated voltage level (VDC) for a solar array.
According to conventional P&O techniques the array's average power is sampled during the positive and negative test pulse periods and combined to form a following error term which has both magnitude and direction. The following error term can be used to adjust the MPP VDC by a particular amount, in either a negative or positive direction, and thus set the MPP VDC for an upcoming period of time.
However, using the P&O method alone can lead to inaccurate results when there is a significant power change due to irradiation, since the power change caused by the test pattern will become swamped by the effect of the power change due to irradiation, giving a false following error value. This can lead to the MPP tracker changing MPP VDC in the wrong direction.
Changes in irradiation which can lead to a significant power change in a solar array are common. Therefore conventional systems which use P&O control for MPP tracking are prone to error. This reduces the overall efficiency and cost effectiveness of the solar array.
According to an aspect a method is provided for determining a value for an electrical output of a converter of renewable energy. The method comprises obtaining a signal representing the electrical output of the converter, wherein that electrical output has an initial value, and applying a pulse signal such as a modulated test pattern to the signal representing the electrical output wherein the pulse signal includes at least a first positive portion and a second negative portion. The positive and negative portions should preferably be of equal magnitude. The method further comprises obtaining a measurement of electrical power produced by the converter during application of the pulse signal and then removing the pulse signal from the signal representing the electrical output. The method further comprises obtaining a measurement of electrical power produced by the converter in the absence of the pulse signal and using said measurements of electrical power to obtain an error value wherein said error value is applied to the initial value of the electrical output of the converter in order to obtain a new target value for the electrical output of the inverter.
Because measurements of electrical power produced by the converter are obtained both during application of the pulse signal and in the absence of the pulse signal, the effects of ambient conditions such as the changes in level of irradiation can be taken into account when determining a target value for the electrical output of the inverter.
The method can further comprise controlling the electrical output of the converter to be equal, or be as close as possible, to a predetermined initial value before the pulse signal is applied and/or controlling the value for the electrical output of the converter to be equal, or be as close as possible, to the target value that has been determined. Therefore the operation of the converter, such as a solar array, can be controlled in accordance with the target value which has been determined.
According to an aspect an electrical power generating system is provided. The electrical power generating system comprises a converter of renewable energy into electrical power and includes means for providing a signal indicative of the electrical power available from the converter, means for providing a signal indicative of the voltage output by the converter, means for applying a pulse signal to the signal indicative of the voltage output by the converter and determining means for calculating a target value for the voltage output by the converter.
According to an aspect a method is provided for determining a target voltage value for a solar array. The method comprises setting the voltage level for the array to an initial value, applying a modulated test pattern to the output voltage level for the array, wherein the modulated test pattern comprises a positive pulse, a negative pulse and a zero pulse (during which the test pattern is effectively absent). The method comprises obtaining a positive perturbation value for the positive pulse of the test pattern, obtaining a negative perturbation value for the negative pulse of the test pattern, and obtaining an irradiation power change value representing a change in power due to irradiation during the zero portion of the test pattern. The method further comprises obtaining an error value from the difference between the irradiation power change value and a combination of the positive and negative perturbation values. Optionally, the method can further comprise applying said error value to the initial value of the output voltage for the array to determine a new target value and, optionally, controlling the output voltage of the array to equal or be as close as possible to that new target voltage.
Because both perturbation values and an irradiation power change value are used in the calculation of the error value, a conventional perturb and observe (P&O) technique is combined with a consideration of the effect of change in irradiation, or exposure to the sun, on the operation of a solar array. As a result the operation of the solar array can be controlled in order to be as efficient as possible.